1. Field of the Invention
The present invention relates to an optical writing type electro-optical device for writing information into an optical modulation member in accordance with an optical signal under application of a voltage.
2. Discussion of the Related Art
As described in Japanese Unexamined Patent Publication Nos. Sho. 59-216126 and Sho. 49-90155, conventional optical writing type electro-optical devices have a cell structure consisting of an optical modulation member (liquid crystal layer) and a photoconductive layer that is homogeneous in a plane expanse. This type of electro-optical device employs amorphous silicon hydride (a-Si:H) for the photoconductive layer, and utilizes its photoelectric effect. In this cell structure, the dark conductivity of the photoconductive layer is approximately equal to the conductivity (10.sup.-11 .OMEGA..sup.-1 cm.sup.-1) of the liquid crystal layer. Therefore, even in a dark state, the impedances of the two layers are approximately equal to each other, to cause a considerable voltage to be applied to the liquid crystal layer. On the other hand, in a bright state, the impedance of the photoconductive layer decreases to be lower than that of the liquid crystal layer, so that an almost full voltage is applied to the liquid crystal layer. Therefore, the electro-optical device of the above structure cannot produce high-contrast images because of a small difference between on- and off-voltages (bright and dark voltages) of the liquid crystal layer.
To solve the above problem, there is a proposal in which the photoconductive layer is constructed so as to have a diode structure. Utilizing the phenomenon that the impedance of this photoconductive layer becomes high when reversely biased in a dark state, the proposal device is so designed that almost no voltage is applied to the liquid crystal layer in a dark state, to thereby obtain a large on/off (bright/dark) voltage difference of the liquid crystal layer. However, when an AC voltage is applied to the device having the diode-type photoconductive layer, the voltage applied to the liquid crystal layer depends on the polarity of the AC voltage because the diode structure shows different characteristics in the forward and reverse directions. This results in an improper operation of the liquid crystal layer.
In addition, conventional optical writing type electro-optical device have a problem that a thick photoconductive layer is needed for the impedance matching between the liquid crystal layer and the photoconductive layer.